Chemical refining of light oil distillates



Jan. 13, 1953 F. A. AEGAR CHEMICAL REFINING OF LIGHT OIL. DISTILLATES Filed April `l5, 1950 Qu Qk NC@ x t fl INVENTOR M wcm G N P um T A. M M.

Patented Jan. 13, 1953 CHEMICAL REFINING OF LIGHT OIL DISTILLATES Frank A. Apgar, Chicago, Ill., assignor to Sinclair Rening Company, New York, N. Y., a corporation of Maine Application April 15, 1950, Serial No. 156,209

4 Claims.

My invention relates to improvements in the sweetening of sour light oil distillates by treatment with copper chloride. In particular, my invention relates to an improved method for sweetening and desulfurizing mercaptan-sour light oil distillates by treatment with copper chloride to convert the mercaptans present to disuldes and to a simplied and more economical elimination of the bulk of the disuldes from the treated distillate.

Light oil distillates produced as petroleum solvents and light naphthas usually contain mercaptans. These mercaptans give the distillates an obJectional odor and must either be removed or converted to a non-objectionable form, namely disuldes, by a sweetening process. A particularly advantageous method Ior sweetening mercaptan-sour light oil distillates is to treat the sour distillate with copper chloride so as to convert the mercaptans to corresponding disuldes which have no obJectionable odor in contrast to lmercaptans from which they were formed and which show no reaction in the copper strip corrosion test ASTM DBG-49T. Stocks containing disuldes of limited concentrations are acceptable to many consumers. The trend in the solvent trade, however, is to demand finished stocks which meet the distillation ask corrosion test in which the copper strip is placed in a flask containing the treated stock which is then distilled to end point. Under the conditions of this test, the disulfides present split up to give corrosive substances which blacken the copper strip. Thus it has become necessary to not only convert mercaptans to disuldes in solvent iinishing, but to remove the disuldes from the treated stock. In conventional operation, the removal of disulfides is exceedingly difficult and time consuming because the dissociation temperature of the disuldes formed in the sweetening operation is normally below the end point of the nished stock. On redistillation of the treated stock to the end point of the untreated stock, the disuldes dissociate in part to mercaptans which are distilled with the overhead stream so that the sweetening operation is nullied to a large extent. If the mercaptan content oi' the overhead stream is sufcient to produce a poor copper strip' or distillation flask corrosion test after reconversion to disuldes, the stock must be again treated and redistilled as before until the final product meets specifications. The conventional procedure requires a large amount of intermediate tankage, is time consuming and reduces net thro'ughput'of existing equipment.

According to my invention, the residence time of disulildes formed in the sweetening operation is reduced in the distillation operation to such an extent that relatively little dissociation of the disuldes to mercaptans can take place. I have found that including a quantity of high boiling hydrocarbons in the treated distillate so as to provide a bottoms fraction in the distillation step which boils above the specication end point of the stock under production and continuous removal of the bottoms fraction during distillation effects a substantial removal of the disuldes before dissociation occurs. Thus my invention provides for production of a specification stock in one pass and eliminates the necessity of re-treating the stock, as is necessary in conventional operation to meet the distillation flask corrosion test. The residence time of the disulndes and consequently the mercaptan content of the overhead product are reduced by increasing the amount of higher boiling hydrocarbons in the untreated stock boiling higher than the end point of the desired product. Advantageously, the quantity of higher boiling hydrocarbons approximates about 8 or 10 to about 20 per cent on the feed, and the material may comprise either a tail on the stock or extraneous bottoms.

My invention will be further illustrated by reference to the accompanying drawing which represents a diagrammatic flow plan.

In the drawing a mercaptan-sour light oil distillate is charged to the sweetening operation by pump III and line Il through heat exchanger I2. Heat exchanger I2 may operate as either a preheater or a cooler in order to better control operation in the primary copper chloride contact tower I3. The sour feed stock may be introduced either to the bottom of the copper chloride tower I3 as by line I5 and dispersing means I6 or to the top of the tower if desired. Air is introduced to the bottom of the copper chloride tower I3 by means of connection II. The treated stock is removed from the top of tower I3 through valved line I9. The treated stock is passed through air release tower 20 from which air is released overhead through line 2l, and the de-aerated stock is removed at the bottom through line 22. The stock is passed through heater 23, which may be heated by 15G-pound steam circulation, into solvent rerun tower 24. The disuldes produced in the copper chloride treating tower I3 are removed with a bottoms fraction withdrawn through line 25 into drum 26, which may be employed as a reboiler drum employing steam as the heating medium. Lighter material is returned to the solvent rerun tower Z through vapor return line 28, While the bottoms and disuldes are withdrawn from the system through line 2l. The

vtreated. and redistilled stock is withdrawn overhead from the solvent rerun tower 24 through line'29 and cooler 30 to accumulator 3I, which is vented to the atmosphere. Advantageously, the accumulator temperature is maintained high enough so that the vapor pressure of the stock is slightly greater than atmospheric. Consequently, a small amount of vapor escapes to the atmosphere and prevents air from backing up through the accumulator 3| to the condensers. Contacting air with the overhead stream is undesirable because it may result in oxidation ofany hydrogen sulfide present from disuldes' dissociating to elementary sulfur, which would give a bad copper strip test ontheV product. A por-i tion of the stock from accumulator 3i may be returned to the solvent rerun tower 2d as reflux through line 32. The bull; of the` stock, however, is passed through line 33 and cooler 34 into caustic wash tower 35. In order to obtain better mixing, the stock is advantageously injected into tower 35 through an eductor and spray nozzle system 36. Caustic recirculation is eifected through line 3'! and the eductor. The washed and separated stock is taken overhead through line 38 to rock salt tower 39, which is packed with rock salt and is designed to remove entrained moisture and caustic soda solution. The dried stock is removed to a secondary copper chloride treating tower 43 through line 42. Air for regenerating the copper chloride may be admitted as by line si at the foot of the rock salt tower S9 or at the foot of the secondary tower 43, or the operation may be conducted in a split tower in which the bottom half or the tower is packed with rock salt and the top half the copper chloride treating mixture. Dispersai aids i and 44 may be provided in contact towers 39 and d3. .il

The finished, sweetened stock is removed to tankage through line d5.

The feed stock o the sweetening operation contains mercaptans in the boiling range of the stock but should be clear, i. e., moisture free and 1.-?

free of hydrogen suliide and elementary sulfur. Advantageously, therefore, the stock is subjected to a preliminary sodium hydroxide solution wash, followed by passage of the stock through a salt tower. The stock to be treated should be paraiiinic' or aromatic in nature and iree of olefins. According to my invention, it must contain a sufcien't quantityV of hydrocarbon material boiling above the' end point of the desired nished product to permit removal of the bulk of the disulfides with the bottoms fraction in the solvent rerun tower so as to reduce the residence time of the disuldes in the distillation tower to a minimum and thereby eiiecti'vely limit re-iormation of mercaptan's by dissociation.

In the copper chloride treating tower, the mercaptans are converted to corresponding disuliides by oxidation with copper chloride. By adding oxygen to the chemicals in the treating tower', the resulting cuprous chloride may be reverted to the cupric form. To improve the operation, it is advantageous to supply a powdered or granular water-absorbent, chemically inert material oi sufficient structural strength to permit handling with the copper chloride in the treating tower. For example, dry Olmsted earth of about 16 to 30 mesh is suite. le. It also is highly desirable to slurry the copper chloride with water before mixing with the adsorbent carrier. If the copper chloride and adsorbent carrier are mixed dry, the materials in the treating bed tend to be hydraulically classiied, with the copper chloride particles generally rising to the top of the bed. Copper carryover or entrainment occurs with subsequent corrosion problems in the equipment. I have found that an especially satisfactory charge to the treating tower consists of water, copper chloride and adsorbent carrier in the approximate weight ratio of 1:1:4.

Since Water is formedl in the regeneration reaction, it is necessary to remove Water from the system to maintain a satisfactory water ratio in the copper chloride bed. If the Water content of the copper chloride charge is appreciably below this amount, copper carryover begins; if the water content rises appreciably above this amount, the copper chloride charge tends to "mud up" or channeling results with formation of copper mercaptides as a result of incomplete conversion' of the mercaptans to the disulfides. The formation of copper mercaptides results in copper carryover with subsequent severe corrosion difficulties.

Water removal is effected by its solubility in the stock being treated Control of the water ratio in the copper chloride bed is maintained by controlling the temperature of the stock in the treating tower within the range approximating to 115 F. by means of appropriate preheating or precooling. The temperature of the stock undergoing treatment is important because the solubility of the water in the hydrocarbon stock` and thus the rate of removal from the system is a function of temperature. Thus, if the bed becomes to wet, the temperature should be raised, Whereas if the bed is too dry, the temperature of the feed stock' should be lowered. A convenient control test is by observation of copper carryover through reaction with diphenylthiocarbazone reagent. For example, when a positive copper test is observed, the preheat may be raised 5 F. If the copper test is negative in a short time, e. g., 1 to 2 hours, it is apparent that the bed was too wet and the increase in preheat should be at least partially maintained. If the copper test is still positive, the bed is too dry and the stock should be precooled. Generally, I have found that the feed should be preheated 3 to 5 F. fora mercaptan-sulfur content of 7 mg. per cc. for the feed stock.

As noted above, oxygen is required for in situ regeneration of the copper chloride treating material, and airis advantageously used for this purpose. Approximat'elyY 1X2 mole of oxygen is required for 2 moles of rnercaptan reacted. The oxygen required per barrel treated per hour approximates 0.015v cubic foot per I mg. of mercaptan sulfur per 100 c`c'. of sour stock. Thus, for a '-mg. mercaptan-sul'fur stock, the air required is equal' to 0.015 5 7 or 0.525 cubic foot of air per barrel per hour. Actually, contact is only about 2U per cent eiicient, so that I recommend about 2.5 cubic feet of air per barrel per hour for a 7-mg. mercaptaI'i-sulfur stock. Also, it is advantageous to employ an excess of air in order to immediately react with hydrogen chloride liberatedl in the treating reaction, so as to assure regeneration of the copper chloride particles and to prevent corrosion difficulties in the subsequent equipment.

In operation, it is desirable to release excess air from the treated stock before redistillation to avoid the possibility of obtaining explosive hydrocarbon and air mixtures in the distillation stage. The stock from the air release tower is preheated and sufficient bottoms are continuously removed in the distillation stage to reduce the residence time of the disulfides in the distillation zone and thus reduce the amount of disulfide breakup. Since a small amount of dissociation may occur even with continuous removal of a large proportion of the stock as bottoms, the redistilled stock is washed with dilute aqueous caustic soda, e. g., about 25 B., to remove any hydrogen sulfide that may have been been formed by the dissociation. Entrained moisture or caustic solution is then removed in a salt tower and any mercaptans in the overhead product from the distillation stage are converted to disulldes in a secondary copper chloride treating tower. The conditions in the secondary treating tower are substantially the same as those obtaining in the primarytreating tower.

I have found that the quantity of bottoms continuously withdrawn in the distillation stage should be per cent on the feed or more. For example, in treating a 10-mg. mercaptan-sulfur per 100 cc. stock for production of a 310 to 390 F. solvent, the reboiler temperature on the solvent rerun tower was operated at 415 to 425 F. With 1 per cent bottoms withdrawal based on the feed, the mercaptan-sulfur in the overhead product was 2.5 mg. per 100 cc. At an B-per cent bottoms withdrawal, the mercaptan-sulfur in the overhead product was 0.9 mg. per 100 cc. With 20 per cent bottoms withdrawal, the mercaptansulfur in the overhead product was 0.5 mg. per 100 cc. In general, I have found that for a 6 to 7 mg. mercaptan sulfur stock, 10 per cent bottoms withdrawal results in an overhead product containing not more than 0.3 to 0.6 mg. of mercaptan-sulfur per 100 cc. A bottoms withdrawal of more than about 20 per cent based on the feed does not produce a suilicient reduction in mercaptan-sulfur to justify the uneconomic handling of the stock.

The following tests on experimental blends made up from stocks handled in a run producing a 300 to 400 F. solvent, taking 20 per cent bottoms on the feed at a 425 F. reboiler temperature, illustrate the necessity of reducing the disullde content of the nished solvent to less than about 1 mg. per 100 cc. to pass the distillation ash corrosion test.

Hence my invention provides a system for treating raw mercaptan-sour solvent stock in a one-pass operation to a finished stock that will pass both copper strip and distillation flask corrosion tests. The use of intermediate run-down tanks and the necessity of repeated re-treating of the finished stock are eliminated. The amount of disuldes in the nished stock may be controlled by varying the bottoms withdrawal in the distillation stage, and a substantial decrease in the total sulfur content of the stock may be effected.

I claim:

1. In a process for sweetening mercaptan-sour light mineral oil distillates by treatment with copper chloride to convert mercaptans to disulfldes followed by distillation of the treated stock wherein disulfides have dissociation temperatures below the end point of the nished stock, the improvement which comprises adding about 8 to about 20% by weight on the treated distillate charge to the distillation step of high boiling hydrocarbons in the distillate to provide a bottoms fraction in the distillation step boiling above the specication end point of the distillate and continuously removing said bottoms fraction containing disuldes during the distillation step in an amount of at least about 8% by weight on the treated distillate charge to the distillation step.

2. The improvement of claim 1 in which the bottoms fraction represents about 10 to about 20 per cent on the treated distillate charged to the distillation step.

3. The method of sweetening and desulfurizing light mineral oil distillates which comprises contacting a distillate having added therein about 8 to about 20% by weight on the treated distillate charge to the distillation step of hydrocarbons boiling higher than the specication end point of the distillate in a treating step with copper chloride, water and a solid water-absorbent material in the approximate weight ratio 1:1:4 in the presence of air to convert the mercaptans in the distillates to disulides having dissociation temperatures below the end point of the finished stock, removing excess air from the treated distillate and redistilling the treated distillate to recover the distillate of specication end point as an overhead product while continuously removing the higher boiling hydrocarbons containing disulfides as a bottoms fraction in an amount of at least about 8% by weight on the treated distillate charge to the distillation step.

4. The method of sweetening and desulfurizing light mineral oil distillates which comprises contacting a distillate having added therein about 8 to about 20% by weight on the treated distillate charge to the distillation step of hydrocarbons boiling higher than the speciiication end point of the distillate in a treating step with copper chloride, Water and a solid water-absorbent material in the approximate weight ratio 1:1:4 in the presence of air to convert the mercaptans in the distillates to disuldes having dissociation temperatures below the end point of the finished stock, removing excess air from the treated distillate, redistilling the treated distillate to recover the distillate of specification end point as an overhead product While continuously removing the higher boiling hydrocarbons containing disuldes as a bottoms fraction in an amount of at least about 8% by weight on the treated distillate charge to the distillation step, washing the redistilled distillate with aqueous caustic soda, removing moisture from the washed distillate by contacting with rock salt and recontacting the distillate in a second treating step with copper chloride, water and a solid waterabsorbent material in the approximate weight ratio 1:1:4 in the presence of air.

FRANK A. APGAR.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Fry et al. Sept. V29, 1942 

1. IN A PROCESS FOR SWEETENING MERCAPTAN-SOUR LIGHT MINERAL OIL DISTILLATES BY TREATMENT WITH COPPER CHLORIDE TO CONVERT MERCAPTANS TO DISULFIDES FOLLOWED BY DISTILLATION OF THE TREATED STOCK WHEREIN DISULFIDES HAVE DISSOCATION TEMPERATURES BELOW THE END POINT OF THE FINISHED STOCK, THE IMPROVEMENT WHICH COMPRISES ADDING ABOUT 8 TO ABOUT 20% BY WEIGHT ON THE TREATED DISTILLATE CHARGE TO THE DISTILLATION STEP OF HIGH BOILING HYDROCARBONS IN THE DISTILLATE TO PROVIDE A BOTTOMS FRACTIONS IN THE DISTILLATION STEP BOILING ABOVE THE SPECIFICATION END POINT OF THE DISTILLATE AND CONTINUOUSLY REMOVING SAID BOTTOMS FRACTION CONTAINING DISULFIDES DURING THE DISTILLATION STEP IN AN AMOUNT OF AT LEAST ABOUT 8% BY WEIGHT ON THE TREATED DISTILLATE CHARGE TO THE DISTILLATION STEP. 